Various methods are known in the art for providing different types of Layer 2 network service over a common packet network infrastructure. (The term “Layer 2” as used herein refers to the second layer in the protocol stack defined by the well-known Open Systems Interface (OSI) model, also known as the logical link, data link, or media access control (MAC) layer.) For example, Malis et al. describe a protocol that can be used to transport Synchronous Optical Network (SONET) frames over a packet network in an Internet Engineering Task Force (IETF) draft entitled “SONET/SDH Circuit Emulation over Packet (CEP)” (draft-ietf-pwe3-sonet-00.txt, July, 2002), which is incorporated herein by reference. This document, along with other IETF documents cited hereinbelow, is available at the IETF Web site. Traffic based on other Layer 2 protocols, such as such as Frame Relay, Asynchronous Transfer Mode (ATM), Ethernet, Cisco High-level Data Link Control (HDLC) and the Point-to-Point Protocol (PPP), may be transported over a packet infrastructure in a similar manner.
Most recent work on Layer 2 transport over packet networks focuses on encapsulation and transport of Layer 2 frames via tunnels through an Internet Protocol (IP) or Multiprotocol Label Switching (MPLS) network. For example, Martini et al. describe how an “Ethernet Pseudowire (PW)” may be created and used to carry Ethernet frames over an IP or MPLS network in an IETF draft entitled “Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLS Networks” (draft-ietf-pwe3-ethernet-encap-02.txt, February, 2003), which is incorporated herein by reference. This technique enables service providers to offer “emulated” Ethernet services over existing IP or MPLS networks. User nodes on different physical local area networks (LANs) can be joined together through PW connections to define a virtual private network (VPN), which appears to the users to be a single Ethernet LAN. Additional PW types, for creating other types of Layer 2 circuits over IP and MPLS networks, are described in other IETF drafts.
Other protocols have been defined for transporting one type of Layer 2 traffic over a connection through another type of Layer 2 network. For example, Mamakos et al. describe methods for providing PPP facilities over Ethernet in IETF Request for Comments (RFC) 2516, entitled, “A Method for Transmitting PPP Over Ethernet (PPPoE)” (February, 1999), which is incorporated herein by reference.
The methods described above are all directed to supporting like-to-like Layer 2 services, i.e., the service endpoints communicate with one another using the same protocol, even though the traffic between the endpoints may be carried over a network that uses a different protocol. In contrast to these methods, interworking of Layer 2 services enables endpoints using disparate protocols to communicate with one another over the same VPN. This idea is described generally by Sajassi et al., in an IETF draft entitled, “L2VPN Interworking” (draft-sajassi-12vpn-interworking-01.txt, March, 2003), which is incorporated herein by reference.